Turbine wheel



Sept. 3, 1968 ARQY ETA; 3,399,443

EEEEEEEEEEE L I A. ROY ET AL Sept. 3, 1968 TURBINE WHEEL 5 Sheets-Sheet2 Original Filed May 27, 1964 v R. m E Z Z M f 2% $1 2 a 4 a%/ 2 a 0 9Sept. 3, 1968 A. ROY ET AL 3,399,443

TURBINE WHEEL Original Filed May 27, 1964 3 Sheets-She'et 5 g MUHUHHHHlinuu il'u H I u 92 Q m (mu \m m M 92 u n I l WM H TTOFNEY$,

Patented Sept. 3, 1968 3,399,443 TURBINE WHEEL Amedee Roy, Birmingham,and John Martin Corwin, Royal ak,'Mich., assignors to ChryslerCorporation, Highland Park, Mich., a corporation of Delaware Originalapplication May 27, 1964, Ser. No. 370,476, now Patent No. 3,262,674.Divided and this application Apr. 5, 1966, Ser. No. 540,327 7 Claims.(Cl. 29-1563) This application is a division of application Ser. No.370,476, filed May 27, 1964, now Patent No. 3,262,674.

invention relates generally to a gas turbine engine and moreparticularly to a turbine wheel for use therein.

In the past, turbine wheels have been formed so as to be comprisedgenerally of a disc-like body portion having a rim formed thereon orsecured thereto and a plurality of radially directed circumferentiallyspaced blades formed on or secured to the rim portion. In some instancesthe disc body has also been provided with a hub portion for securing thewheel to a cooperating shaft. I

In every application of a turbine wheel one of the prime considerationsis the inertial characteristics of the wheel. That is, in order toassure quick response by the wheel to the motive fluid passingtherethrough, precautions are normally taken to reduce the polar momentof inertia of the wheel. This usually is achieved by reducing the massof the Wheel in somewhat a proportion to the radial distance of thatmass away from the axis of rotation of the wheel.

In gas turbine engine applications, the turbine wheels experiencevarious stresses during normal engine operation. These stresses can bebroadly classified into three general categories the first of whichincludes mechanical stresses due to the centrifugal force resulting fromhigh speed rotation of the turbine wheel. The second category includesthose stresses arising from the vibratory energy induced into theturbine wheel, while the third category of stresses could be referred togenerally as thermostresses arising from exposure of at least theturbine blades to relatively high heats as that occasioned by theextremely hot motive gases passing therethrough.

Becsause of these three categories of stresses the mass of the rims andbodies of turbine wheels of the prior art design could not beeffectively reduced to the degree desired in order to obtain a highresponsive turbine wheel. That is, heretofore it has been considerednecessary to have the rim and disc body adjacent the rim of across-sectional thickness sufficient to prevent cracking of the turbinewheel due to the stresses developed during operation. The crossseetionalthickness in such instances has in turn caused the general peripheralmass of the wheel to increase to the degree resulting in a considerableincrease in the inertia and consequent loss in wheel accelerationresponse.

Accordingly, an object of this invention is to provide a novel andimproved turbine wheel which has a relatively low polar moment ofinertia.

Another object of this invention is to provide a turbine wheel of aconfiguration which effectively eliminates the deleterious effects ofthermal stresses normally arising from exposure to relatively hot motivegases.

Still another object of this invention is to provide means in a turbinewheel for effectively reducing or minimizing the occurrence of damaginginduced vibratory stresses.

Other objects and advantages of the invention will become apparent whenreference is made to the following description and accompanying drawingswherein:

FIGURE 1 is a side elevational view of a turbine wheel constructed inaccordance with the teachings of this invention;

FIGURE 2 is an end view of the turbine wheel taken generally in thedirection of arrow A of FIGURE 1;

FIGURE 3 is an enlarged fragmentary elevational view taken substantiallyon line 3-3 of FIGURE 1;

FIGURE 4 is an enlarged fragmentary elevational view taken substantiallyon line 4-4 of FIGURE 1 and looking in the direction of the arrows;

FIGURE 5 is a-fragmentary elevational View taken generally on line 5-5of FIGURE 2;

FIGURE 6 is an enlarged fragmentary cross-sectional view taken generallyon the plane of line 6-6 of FIG- URE 4, also illustrated in FIGURE 7,andlooking in the direction of the arrows; v

FIGURE 7 is an enlarged fragmentary cross-sectional view taken generallyaxially of the turbine wheel as, for example, on the plane of line 77 ofFIGURE 3;

FIGURE 8 is a fragmentary view similar to FIGURE 3, diagrammaticallyillustrating the forces incurred by an infinitesimal particle of theturbine wheel during peri ods of operation;

FIGURE 9 is an enlarged fragmentary elevational view of a portion of theturbine wheel taken generally on the plane of lines 99 of both FIGURES 5and 7; 1

FIGURE 10 is a cross-sectional view, similar to FIG- URE 6, takengenerally on the plane of line 10-10 of FIGURE 9;

FIGURE 11 is an enlarged fragmentary cross-sectional view illustratingin greater detail a portion of the preferred embodiment of thisinvention;

FIGURE 12 illustrates, generally, apparatus suitable for use in forminga turbine wheel in accordance with this invention; and

FIGURE 13 is an enlarged fragmentary view, in cross section, of aportion of the apparatus of FIGURE 12.

Certain details are omitted from one or more figures for purposes ofclarity.

Referring now in greater detail to the drawings, a turbine wheel 10 isillustrated as being comprised generally of a disc-like body 12 providedwith a centrally disposed hub portion 14 for mounting the body 12 to asuitable shaft, and a plurality of radially directed circumferentiallyspaced blades 16 against which a cooperating motive fluid is directed inorder to impart rotative motion to the turbine wheel 10 and itsassociated shaft. Blades 16 are formed as to extend generally radiallyoutwardly from the outer surface 18 of rim 20 which comprises a portionof an annular support or pedestal arrangement 22.

The annular support 22 is comprised of a plurality of generally radiallydirected circumferentially spaced front and rear struts or ribs 24 and26, respectively. The terms front and rear are used to denote theposition of the ribs axially of the wheel 10. That is, the front ribs 24would be on the upstream side of the wheel while the rear ribs 26 wouldbe on the downstream side of the wheel.

Succeeding ribs or struts are joined to each other at their respectiveradially innermost ends by a fillet-like arcuate portion 28 as showngenerally in FIGURES 2, 3 and 4. The radially outermost ends of each ofthe ribs is provided with a tapered portion 30 which tapers outwardlyfrom the rib and joins a similarly tapered portion of the adjacent ribto form an apex or juncture 32 which is generally conterminous with thelower portion of the rim 20.

Surfaces 34 and 36 of adjacent front ribs 24 along with tapered portions30 and arcuate portions 28 collectively define front recesses 38 whichextend inwardly and generally axially of the wheel 10. Similarly,surfaces 34 and 36 of adjacent rear ribs 26 along with associatedtapered portions 30 and arcuate portions 28 collectively define rearrecesses 40 which also extend inwardly and generally axially of thewheel. As illustrated in FIGURES 6 and 7, the front and rear recessesextend inwardly of the wheel 10 towards each other and are separated bya common thin wall 42.

It should be noted that both recesses 38 and 40 extend inwardly into thewheel 10 at a slight angle with respect to a plane containing the axisof the wheel. For example, a plane passing through the middle of recess38 will have a trace 48 at an angle Y with respect to trace 50 of theplane passing through the axis of wheel 10. Similarly, a plane passingthrough the middle of recess 40 will have a trace 52 which is at anangle X with respect to trace 50. In the embodiment disclosed, angle Xis greater than angle Y; however, as will become apparent the preciserelationship of angles X and Y is not controlling in the practice of theinvention.

Both sides of each of the walls 42 are preferably tapered inwardlytowards each other so as to form indentations 44 and 46 generally alongthe wall 42 and radially of the wheel 10. Indentations 44 and 46 areprovided in order to define an area of reduced cross-sectional thicknessin walls 42 thereby creating an area for stress concentration. Incertain of particularly successful embodiments of the invention thethickness of the reduced portion of the Walls 42 was in the order of0.010 to 0.020 inch.

In FIGURE 6 the root profile of a couple of blades 16 is illustrated inphantom line in order to better illustrate the position of therespective blades to the ribs 24 and 26 immediately radially inwardly ofthe rim 20.

Referring to FIGURES 3, 4 and 6, if recesses 38 are viewed in thedirection of arrow B of FIGURE 6, it can be seen that the outermost endof the respective apexes 32 are substantially midway of the projecteddistance between root 54 of the front of one of the blades 16 and root56 of a median portion of the next adjacent blade 16. Similarly, ifrecesses 40 are viewed in the direction of arrow C of FIGURE 6, it canbe seen that the outermost end of the respective apexes 32 of recesses40 are substantially midway of the projected distance between root 58 ofthe rear of one of the blades 16 and the root '56 of the median portionof the next adjacent blade 16.

In FIGURE 6, the forward and rearward ends of the respective blades areshown as projecting some distance beyond ribs 24 and 26. Such ends are,however, fully supported by the respective ribs because, as illustrated,for example, in FIGURES 3, 4 and 5, the ribs are provided with not onlytapered portions 30 but also taper outwardly from the axially medialportion of the wheel so as to have the largest width axially of thewheel immediately below the rib 20. FIGURE 6 is a cross-sectional viewobtained on lines 66 of either FIGURE 4 or 7 somewhat radially inwardlyof where tapered portions 30 join surfaces 34 and 36.

Referring to FIGURES l and 8, let it be assumed that the wheel 10 iscaused to rotate about its axis 60 and that the square, identified as P,is an infinitesimal particle of the wheel. Due to the centrifugal andcentripetal forces developed during rotation of wheel 10, particle Pexperiences tension as illustrated generally by the force vectors 62 and64. That is, particle P is urged radially outwardly of wheel 10 but atthe same time restrained to some degree by the adjoining particles. Thegreater the angular velocity, the greater, of course, are forces 62 and64.

With the high rotational speeds experienced by turbine wheels actualradial expansion of the wheel is incurred. Accordingly, it can beappreciated that such radial expansion requires substantially everyinfinitesimal particle of the wheel to experience slight radiallyoutward movement.

Consequently, the infinitesimal particles circumferentially 4. merelyrotation of the turbine wheel. However, additional forces are createdwhenever the wheel 10 is exposed to high heat as, for example, the hotmotive gases employed in a gas turbine engine.

Referring to FIGURES 7 and 8, let it be assumed that hot motive gasesare being directed through an annular conduit and against blades 16 soas to impart rotative motion to wheel 10. Due to rotation, particle Pwill, of course, experience forces 62, 64, 66, and 68 as previouslydescribed. However, because of the heat transfer incurred as between thehot motive gases, the blades 16 and pedestal structure 22, the forcesdeveloped on particle P are somewhat altered.

For example, referring to FIGURES 7 and 2, it should be apparent that atemperature gradient will exist as between generally blades 16-and thehub 14 of wheel 10 due to the relatively cold hub 14 and the hot motivegases impinging against blades 16. Consequently, the radially outermostportions such as rim 20 and support structure 22 expand to a greaterextent than the adjoining radially inner portions of the wheel 10.

Therefore, since portion 12 of wheel 10 prevents rim 20 and supportstructure 22 from expanding to the degree normally required by thetemperature of the rim and support, rim 20 and wall 42 of support 22 areplaced in a state of circumferential or tangential compression. Ifparticle P of FIGURE 8 is considered to be a particle of rim 20, forexample, it becomes apparent that because of the attempt to expand bythe circumferentially adjoining particles and the restraining effectthereon by the relatively cooler portion of the whee1,that particle P isnow placed in a condition of circumferential or tangential compressionas indicated generally by force vectors 72 and 74 which have replacedvectors 66 and 68. As the temperature gradient increases the compressionexperienced in the rim 20 becomes sufficient to result in plasticdeformation of the rim material. This deformation accommodates, to somedegree, the expansion required by the relatively hot radially outerportions of the wheel.

However, upon subsequent cooling of the wheel 10, as occasioned during,for example, engine shut-down or reduced temperature and engine output,a reversal of forces occurs. That is, those sections of the radiallyouter portion of the wheel 10, such as rim 20, attempt to contract andreturn to their normal state free of induced stresses. However, suchcontraction is inhibited because these very same portions have, asdiscussed above, previously undergone plastic deformation. Consequently,the particles, such as P, which were previously in a state of tangentialcompression are once more placed in a state of circumferential ortangential tension as illustrated generally by force vectors 66 and 68.These forces in turn cause cracks to occur generally radially of wheel10 at points of greatest stress concentration.

The problem of thermally induced stresses as described above is wellknown to the prior art. It has been a commonly accepted belief, by thoseskilled in the art, that such thermally induced wheel cracks areincompatible to an otherwise properly functioning turbine wheel and thata turbine wheel which developed such cracks could not be further safelyemployed within an engine. Consequently, the prior art has heretoforeproposed various arrangements for strengthening the turbine wheel rimand/or thermally isolating the wheel blades 16 from the supporting rimstructure. Such proposed solutions have not, however, proven to beentirely satisfactory.

In the embodiment of the invention disclosed, not only does rim 20experience the various forces described but so also does the web or Wall42 separating recesses 38 and 40. This invention, contrary to the priorart and contrary to the commonly accepted belief by those skilled in theart, provides an arrangement specifically intended to give rise to theoccurrence of such thermally induced cracks.

As previously stated, the cross-sectional thickness from apexes 32 tosurface 18 of rim 20 are minimal as are the cross-sectional thicknessesof webs 42 between indentations 44 and 46. Accordingly, as compressiveforces are created, due to temperature gradients, plastic deformation,if it is to occur, will exhibit itself to the greatest extent in suchsections of reduced cross-sectional area. Subsequently, as wheel cools,during periods of, for example, engine shutdown, reverse tangentialtensional forces occur on the infinitesimal particles comprising suchsections of reduced cross-sectional area. Consequently, contrary to theprior art, cracks are encouraged to occur through such sections oflocalized stress. As such cracks occur, they will appear through webs orwalls 42 in the vicinity of indentations 44 and 46 so as to assume agenerally radial position with respect to the axis 60 of the wheel 10.Further, cracks will also occur generally axially of wheel 10 throughrim so as to be in a pat tern generally defined by plane traces 48 and52 of FIGURE 6. An example of such a generally axially directed crack isillustrated at 76 of FIGURE 5.

The radial cracks 75 will, of course, occur in webs 42 at the areasgreatest stress concentration which would, in the embodiment of theinvention disclosed, be determined by indentations 44 and 46. Suchcracks 75 progress radially inwardly towards the center of wheel 10 tothe point where provision is made to limit the further progress of suchcracks.

The invention as herein disclosed contemplates the provision of crackbarriers in the form of stress dissipating apertures 78 formed througheach of the webs or walls 42. The provision of such apertures 78prevents the propagation of radial cracks 75 beyond a predeterminedpoint on the wheel 10.

As best seen in FIGURES 7 and 9 the stress relieving barrier 78 is anaperture of generally U-shaped or semicircular configuration. It can beappreciated that as crack 75 progresses to where it breaks through theupper or radially outer surface or edge 80 of aperture 78, that theforces tending to propagate crack 75 are dissipated.

In many instances it has been found to be a decided advantage to providesome means which would effectively seal the stress relieving barrier 78to the passage of gases therethrough and yet not defeat the purpose ofthe aperture 78. This, it has been discovered, can be quickly andeffectively accomplished by the use of, for example, a plasma arc sprayprocess or a flame spray process. It has been found that with either ofthese processes a metallic layer, as indicated generally at 84 of FIGURE11, can be deposited generally about and across the aperture 78 in orderto form a sealing means. In the preferred em bodiment nickel andaluminum would be deposited by either of the above processes to formlayers 84 which would be comprised, principally, of NiAl and Ni AI.

It should be noted that the layer 84 is applied in a manner which willnot allow any crack to be propagated thereacross and into the lower edgeor surface 82 of aperture 78. That is, it has been determined that theparticles of nickel and aluminum cause a metallurgical bond with wall 42because of the exothermic reaction of the particles and the heattransferred thereby upon striking the surface of wall 42. It also hasbeen determined that if the accelerated particles of nickel and aluminumare directed against a surface of, for example, wall 42 in a path whichforms an angle less than 45 with that surface, the heat transfer to thework piece is usually insuiiicient to cause a metallurgical bondthere'between.

Accordingly, the plasma are or flame spray gun, illustratedschematically at 86 of FIGURE 11, is held with respect to wall 42 sothat a metallurgical bond will be effected generally radially outwardlyof surface 80. -It should be noted from the schematic representation ofFIGURE 11 that particles of aluminum and nickel will not bond to thesurfaces 80 and 82 defining aperture 78 because the angle of incidenceof the accelerated particles is less than sufficient to cause therequired metallurgical bonding. This is also true for the surface offillets 28.

Even though the elongated arcuate stress dissipating apertures 78 can beformed by any-suitable process as, for example, by casting, thepreferred method of forming such apertures 78 can be best described withreference to FIGURES 10, 12. and 13.

It has been discovered that the elongated arcuate slots or stressbarriers 78 can be most expeditiously formed by any of the machiningprocesses often referred to as electrical discharge machining. Althoughelectrical discharge machining is relatively well know in the art theuse thereof is employed as one step in a novel method of forming aturbine wheel as disclosed herein.

For example, referring to FIGURE 10; an electrode 88 of a suitableelectrical discharge machine is shown being brought into position forthe machining or meta'lrernoving step of the process. The hidden lines90 mdrcate generally the path of the electrode 88 through wall 42. As iswell known in the art of electrical discharge machming, the endprojection of electrode end 92 is of a configuration and size closelyapproximating the arcuate opening 7 8.

In order to prevent damage to surfaces 34 by electrode 88 as it passesthrough wall 42, cavities or chambers 40 are filled as illustrated witha suitable dielectric material 94. Accordingly, as electrode 88 emergesthrough wall 42 into cavity 40, the path for spark discharge between theend of electrode 88 and surface 34 is closed by the presence of thedielectric material 94. It has been discovered that wax is a highlysuitable dielectric material.

FIGURE 12 illustrates an apparatus whereby all of the arcuate stressbarriers 78 can be formed simultaneously. The apparatus is comprisedgenerally of suitable holding .fixture 96 for the turbine wheel 10(workpiece), and a relatively movable electrode holder 98 which may besuitable mounted on posts 100 slidably received within stationarybushings 102.

Preferably, as illustrated in FIGURE 13, each of the electrodes 88 isprovided with frictionally engaging means, such as a spring 104, forretaining the electrodes 88 within the openings 106 of electrode holder98. The frictional retaining means 104 is such as to at times permitrelative motion between the electrode 88 and holder 98. Such relativemotion is highly useful, [for example, referring to FIGURES l2 and 13,the electrode holder 98 may be lowered until each of the electrodes 88abuts against its respective wall 42. Any variations electrode position(axially) will automatically be eliminated (or if such variations arenecessary they will be created) by the relative motion betweenelectrodes 88 and holder 98 permitted by the frictional retaining meansor spring 104.

As is well known in the art of electrical discharge machining, the endof the electrode is usually held relatively close to the workpiece, at adistance of, for example, in the order of 0.001 inch. Accordingly, oncethe relative axial positions of electrodes 88 are determined the holder98 may be moved upwardly a selected distance thereby establishing thegap between each of the electrodes and corresponding wall 42,simultaneously.

The electrode holder 98 is then fed downwardly at a controlled rateuntil all of the electrodes 88 form their respective arcuate apertures78. 'If the dielectric material 94 is wax or any other dielectricsubstance which has sufficient density or body to resist the furtheraxial move ment of electrodes 88, then it becomes possible to haveelectrodes which will not complete their respective apertures 78. Thatis, because of various reasons, such as the configuration of theworkpiece, the possible variations in the thickness of the wall 42, thepossible variations in the rate of wear or erosion of the electrode 88itself, it is quite conceivable and probable that only a portion of anelectrode will break through wall 42 and contact the dielectric material94. Because of the density of the dielectric material 94 land thefrictional engagement of the electrode 88 with holder 98, further axialmovement of the electrode is prevented. Consequently, the electrode 88would not complete the arcuate aperture 78 because the dielectricmaterial would continue holding the end of the l60tf0id6 88 away fromthe remaining material to be machined from wall 42 regardless of themovement of electrode holder 98.

In order to avoid this possibility, a relatively thin layer ofelectrically conductive material 108 is provided generally between thedielectric material 94 and wall 42. It has been discovered that a layerof aluminum foil is highly suitable for this purpose.

It can be seen that through use of such a conductive material theelectrode 88 will have to penetrate the layer 108 before coming intocontact with the dielectric 94. The extra thickness of conductivematerial provided by layer 108 assures that the electrode will completeits aperture 78 prior to engaging the dielectric 94.

The invention as disclosed, among other things, provides simple and yethighly elfective means for limiting radial crack propagation in aturbine wheel. Further, the particular crack limiting means is such asto lend itself to a novel method of manufacture which, in turn, greatlyreduces the cost of the turbine wheel.

Although only a preferred embodiment of the invention has been disclosedand described, it is apparent that other embodiments and modificationsof the invention are possible within the scope of the appended claims.

We claim:

1. A method of constructing a turbine wheel which is highly responsiveto changes in the rate of flow of motive driving fluid through theblades thereof and which elfectively prevents the formation ofundesirable random cracks developed by repeated cyclic exposure of theturbine wheel blades to hot motive gases, comprising the steps offorming a turbine wheel to have a disc-like body with an annularpedestal portion radiating therefrom and circumferentially thereabout,forming a plurality of circumferentially spaced turbine wheel bladesabout said pedestal portion so as to be directed radially outwardlytherefrom, forming a plurality of generally axially directed radiallyextending circumferentially spaced recesses in said pedestal portionthereby defining a plurality of generally axially directed radiallyextending struts therebetween, forming each of said recesses so as tohave the radially inner-most surface thereof of generally arcuateconfiguration joining the radially innermost ends of pairs of successivestruts, forming an annular axially directed rim so as to join theradially outermost end of each of said struts, forming a tapered portionon each side of each of said struts near the radially outermost endthereof in a manner causing said tapered portions of succeeding strutsto form a juncture at a point which is substantially on the radiallyinnermost surface of said rim thereby providing first portions forlocalizing stresses, forming a web in each of said recesses so as tojoin succeeding struts thereby dividing said recesses into upstreamrecesses and downstream recesses and effectively preventing the passageof motive fluid between said struts, forming sec ond portions forlocalizing stresses formed on said webs, placing a layer of electricallyconductive material within at least one of said upstream recesses andagainst said web therein, at least partially filling said one of saidupstream recesses with a dielectric material, placing an electrode ofgenerally arcuate cross-section partially into one of said downstreamrecesses which is in general juxtaposition to said one of said upstreamrecesses, applying an electrical potential to said electrode and saidweb between said one upstream and said one downstream recess therebycausing electrical discharges as between said electrode and said web inorder to remove material from said web, advancing said electrode towardsaid web as said material is being removed, continuing to advance saidelectrode until said electrode abuts against said dielectric materialthereby forming an aperture through said web, and withdrawing saidelectrode from said downstream recess.

2. A method of constructing a turbine wheel according to claim 1including the step of directing a spray of metallic particles againstsaid web so as to form a bridging layer across that portion of said webwhere said material was removed by said electrode.

3. A method of forming an aperture through a wall portion of a metallicarticle, comprising the steps of placing a thin layer of electricallyconductive material on one side of said wall portion, placing a layer ofdielectric material against said layer of electrically conductivematerial so as to generally contain said electrically "conductivematerial between said one side of said wall portion and said dielectricmaterial, placing electrical discharge machining electrode in closeproximity to the other side of said wall portion opposite to said oneside, applying an electrical potential to said wall portion and to saidelectrode thereby causing electrical discharges as between said wallportion and said electrode in order to remove material from said wall,advancing said electrode toward said wall at a rate related to the rateat which said material is being removed, continuing to advance.

said electrode until said electrode passes through said wall portion andat least in part passes through said layer of electrically conductivematerial to abut against said dielectric material, said dielectricmaterial serving to prevent the occurrence of electrical discharges asbetween said electrode and a portion of said metallic article other thansaid wall portion.

4. A method of forming an aperture through a wall portion of a metallicarticle according to claim 3, wherein said dielectric material compriseswax.

5. A method of forming an aperture through a wall portion of a metallicarticle according to claim 3, wherein said layer of electricallyconductive material comprises aluminum.

6. A method of forming an aperture through a wall portion of a metallicarticle according to claim 3, wherein said dielectric material compriseswax and said layer of electrically conductive material comprisesaluminum.

7. A method of forming a plurality of apertures through wall means of ametallic article, comprising the steps of placing a thin layer ofelectrically conductive material on one side of said wall means, placinga layer of dielectric material against said layer of electricallyconductive material so as to generally contain said electricallyconductive material between said one side of said wall means and saiddielectric material, mounting a plurality of electrical dischargemachining electrodes within a suitable holder in a manner permitting ofrelative motion between said holder and electrodes upon an applicationof sufiicient force against said electrodes, creating relative motionbetween said holder with said electrodes and said metallic article so asto cause at least certain of said electrodes to abut against the otherside of said wall means opposite to said one side, continuing to createrelative motion of said holder toward said article so as to enable saidcertain electrodes to experience relative motion with respect to saidholder while causing the remaining electrodes to abut against the saidother side of said wall means in order to generally conform todimensional variations in said wall means, moving said holder with saidelectrodes and said article away from each other so as to provide aslight clearance between the respective ends of said electrodes and saidwall means, applying an electrical potential to said wall means and saidelectrodes thereby causing electrical discharges as between said wallmeans and said electrodes in order to remove material from said wallmeans, advancing said holder and electrodes toward said wall means at arate related to the rate at which said material is being removed,continuing to advance said holder and electrodes until said electrodespass through said wall means and at least in part pass through saidlayer of electrically conductive material to abut against saiddielectric material, said dielectric material serving to prevent theoccurrence of electrical dis- 10 charges as between said electrodes andportions of said 3,098,148 7/1963 Plot et a1 219-69 metallic articleother than said wall means. 3,148,446 9/ 1964 Celovsky 29558 3,314,1374/1967 Schellens 29558 References Cited UNITED STATES PA 5 JOHN F.CAMPBELL, Primary Examiner. 2,786,128 3/1957 Lines 219-69 P. M. COHEN,Assistant Examiner.

2,909,641 10/1959 Kucyn 21969

1. A METHOD OF CONSTRUCTING A TURBINE WHEEL WHICH IS HIGHLY RESPONSIVETO CHANGES IN THE RATE OF FLOW OF MOTIVE DRIVING FLUID THROUGH THEBLADES THEREOF AND WHICH EFFECTIVELY PREVENTS THE FORMATION OFUNDESIRABLE RANDOM CRACKS DEVELOPED BY REPEATED CYCLIC EXPOSURE OF THETURBINE WHEEL BLADES TO HOT MOTIVE GASES, COMPRISING THE STEPS OFFORMING A TURBINE WHEEL TO HAVE A DISC-LIKE BODY WITH AN ANNULARPEDESTAL PORTION RADIATING THEREFROM AND CIRCUMFERENTIALLY THEREABOUT,FORMING A PLURALITY OF CIRCUMFERENTIALLY SPACED TURBINE WHEEL BLADESABOUT SAID PEDESTAL PORTION SO AS TO BE DIRECTED RADIALLY OUTWARDLYTHEREFROM, FORMING A PLURALTIY OF GENERALLY AXIALLY DIRECTED RADIALLYEXTENDING CIRCUMFERENTIALLY SPACED RECESSES IN SAID PEDESTAL PORTIONTHEREBY DEFINING A PLURALITY OF GENERALLY AXIALLY DIRECTED RADIALLYEXTENDING STRUTS THEREBETWEEN, FORMING EACH OF SAID RECESSES SO AS TOHAVE THE RADIALLY INNERMOST SURFACE THEREOF OF GENERALLY ARCUATECONFIGURATION JOINING THE RADIALLY INNERMOST ENDS OF PAIRS OF SUCCESSIVESTRUTS, FORMING AN ANNULAR AXIALLY DIRECTED RIM SO AS TO JOIN THERADIALLY OUTERMOST END OF EACH OF SAID STRUTS, FORMING A TAPERED PORTIONON EACH SIDE OF EACH OF SAID STRUTS NEAR THE RADIALLY OUTERMOST ENDTHEREOF IN A MANNER CAUSING SAID TAPERED PORTIONS OF SUCCEEDING STRUTSTO FORM A JUNCTURE AT A POINT WHICH IS SUBSTANTIALLY ON THE RADIALLYINNERMOST SURFACE OF SAID RIM THEREBY PROVIDING FIRST PORTIONS FORLOCALIZING STRESSES, FORMING A WEB IN EACH OF SAID RECESSES SO AS TOJOIN SUCCEEDING STRUTS THEREBY DIVIDING SAID RECESSES INTO UPSTREAMRECESSES AND DOWNSTREAM RECESSES AND EFFECTIVELY PREVENTING THE PASSAGEOF MOTIVE FLUID BETWEEN SAID STRUTS, FORMING SECOND PORTIONS FORLOCALIZING STRESSES FORMED ON SAID WEBS, PLACING A LAYER OF ELECTRICALLYCONDUCTIVE MATERIAL WITHIN AT LEAST ONE OF SAID UPSTREAM RECESSES ANDAGAINST SAID WEB THEREIN, AT LEAST PARTIALLY FILLING SAID ONE OF SAIDUPSTREAM RECESSES WITH A DIELECTRIC MATERIAL, PLACING AN ELECTRODE OFGENERALLY ARCUATE CROSS-SECTION PARTIALLY INTO ONE OF SAID DOWNSTREAMRECESSES WHICH IS IN GENERAL JUXTAPOSITION TO SAID ONE OF SAID UPSTREAMRECESSES, APPLYING AN ELECTRICAL POTENTIAL TO SAID ELECTRODE AND SAIDWEB BETWEEN SAID ONE UPSTREAM AND SAID ONE DOWNSTREAM RECESS THEREBYCAUSING ELECTRICAL DISCHARGES AS BETWEEN SAID ELECTRODE AND SAID WEB INORDER TO REMOVE MATERIAL FROM SAID WEB, ADVANCING SAID ELECTRODE TOWARDSAID WEB AS SAID MATERIAL IS BEING REMOVED, CONTINUING TO ADVANCE SAIDELECTRODE UNTIL SAID ELECTRODE ABUTS AGAINST SAID DIELECTRIC MATERIALTHEREBY FORMING AN APERTURE THROUGH SAID WEB, AND WITHDRAWING SAIDELECTRODE FROM SAID DOWNSTREAM RECESS.
 3. A METHOD OF FORMING ANAPERTURE THROUGH A WALL PORTION OF A METALLIC ARTICLE, COMPRISING THESTEPS OF PLACING A THIN LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL ON ONESIDE OF SAID WALL PORTION, PLACING A LAYER OF DIELECTRIC MATERIALAGAINST SAID LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL SO AS TOGENERALLY CONTAIN SAID ELECTRICALLY CONDUCTIVE MATERIAL BETWEEN SAID ONESIDE OF SAID WALL PORTION AND SAID DIELECTRIC MATERIAL, PLACINGELECTRICAL DISCHARGE MACHINING ELECTRODE IN CLOSE PROXIMITY TO THE OTHERSIDE OF SAID WALL PORTION OPPOSITE TO SAID ONE SIDE, APPLYING ANELECTRICAL POTENTIAL TO SAID WALL PORTION AND TO SAID ELECTRODE THEREBYCAUSING ELECTRICAL DISCHARGES AS BETWEEN SAID WALL PORTION AND SAIDELECTRODE IN ORDER TO REMOVE MATERIAL FROM SAID WALL, ADVANCING SAIDELECTRODE TOWARD SAID WALL AT A RATE RELATED TO THE RATE AT WHICH SAIDMATERIAL IS BEING REMOVED, CONTINUING TO ADVANCE SAID ELECTRODE UNTILSAID ELECTRODE PASSES THROUGH SAID WALL PORTION AND AT LEAST IN PARTPASSES THROUGH SAID LAYER OF ELECTRICALLY CONDUCTIVE MATERIAL TO ABUTAGAINST SAID DIELECTRIC MATERIAL, SAID DIELECTRIC MATERIAL SERVING TOPREVENT THE OCCURRENCE OF ELECTRICAL DISCHARGES AS BETWEEN SAIDELECTRODE AND A PORTION OF SAID METALLIC ARTICLE OTHER THAN SAID WALLPORTION.